When radio frequency signals, especially those transmitted at very high frequencies (i.e., above 800 MHz), are received at a radio receiver, the signals are often accompanied by electrical noise and interfering signals. The traditional approach to eliminating such interference within the radio receiver is to place one or more high-Q filter elements in the path of the received signal. For very high frequency receivers, successful filtering of the interference is difficult. Often, the spectral spread, that is, the difference between the desired signal frequency and that of the interference, is very narrow so that the required skirt selectivity of the filter elements must be very sharp. Unfortunately, the higher the skirt selectivity of such filters, the greater their in-line losses, thus degrading the receiver sensitivity.
One possible approach to solving the problem of eliminating the interference is to employ adaptive canceling whereby a cancellation signal, 180 degrees out of phase with the interference signal, is injected into the signal spectra to cancel the interference. The effectiveness of this technique depends on the degree to which the cancellation signal is completely out of phase with the interference.